No announcement yet.

Re: Isokinetics and Inverse Dynamics

This topic is closed.
  • Filter
  • Time
  • Show
Clear All
new posts

  • Re: Isokinetics and Inverse Dynamics

    Dear all,
    A number of interesting problems is addressed here. It may be good to
    separate them carefully.

    1) In my opinion the most interesting point is the question of
    maximum activation. Paul Devita rightly points to this effect:
    maximum moments in jumping or running are substantially higher than
    those on an isokinetic dynamometer, and even more so, I might add,
    than the so-called Maximum Voluntary Contraction. I am not so
    informed on quadriceps, but in triceps surae (ankle plantarflexors)
    typical peak values are, from a paper of ours in the press:
    175 Nm isometric MVC
    110 Nm in a TWO legged squat jump (one-legged seems to be higher)
    155 Nm in landing after a 2-legged jump
    225 Nm in sprinting

    The point is even worse when it is considered that the muscle is not
    isometric, but shortening at the time of peak moment. By reckoning
    the effects of series-elasticity, we could estimate (rather roughly)
    the peak 'active state' of the triceps surae, and got:
    175 Nm isometric MVC (the same of course)
    270 Nm in a TWO legged squat jump
    150 Nm in landing after a 2-legged jump (eccentric contraction, thus
    not so much higher)
    500 Nm in sprinting!

    In the EMGs the effect is also clearly visible: peak rectified EMG in
    running can be 200% of 'MVC' or more (like Jacobs and Van Ingen
    Schenau, 1992).

    On this point I agree with Ton vd Bogerts remarks sub 3). In the
    big leg muscles, there must be motor units (and probably the
    strongest ones) which become only active in very brief actions at the
    highest levels of activation. In small muscles e.g. in the hand, some
    95% of maximum may be reached. McComas in his book "Skeletal Muscle"
    (Human kinetics, 1996) on p. 211 discusses the problem, but in my
    opinon he underestimates the effect.
    Electrical stimulation is not the solution. In the big leg muscles is
    is very difficult, painful and even dangerous to try maximal
    stimulation. In the subject referred to above (a very tough guy) we
    could reach 170 Nm, the others did not let us go above 120 Nm. (In
    Scandinavia they might reach higher levels. ) I heard a story from a
    well-known investigator who stimulated the knee extensors of a
    colleage to such an extent, that the patella luxated with a loud

    2) The inverse dynamics is quite prone to methodological errors,
    Bogert point 1), but I do not suppose that it is very relevant here.
    Agree with Vasilios Baltzopoulos.

    3) Paul Devita gives a muscle force of 700 Nm. This is incorrect, see
    remarks of Vasilios. You have probably used the moment arm of the
    ergometer force transducer (some 20 cm) instead of that of the
    patellar tendon, 4 cm.

    4) Due to the effects of the series-elastic component, isokinetic
    speed is only equal to shortening speed of the muscle fibres at the
    peak of the moment (when dM/dt = 0). I have investigated this
    problem, and the differences are very substantial. So only a single
    point can be used to construct a force-velocity curve!

    There are some interesting effects here, which surely warrant
    further discussion and investigation.

    At Hof
    Department of Medical Physiology &
    Laboratory of Human Movement Analysis AZG
    University of Groningen
    Bloemsingel 10
    Tel: (31) 50 3632645
    Fax: (31) 50 3632751

    To unsubscribe send SIGNOFF BIOMCH-L to
    For information and archives: